1. Field of the Invention
The present invention relates to a focusing technique.
2. Description of the Related Art
Electronic cameras perform focusing so that a lens's position in which a high-frequency component of a luminance signal of an image of an object formed on an image pickup element is largest is set as a focus position, at which the desired object's image is in focus. The large amount of the high-frequency component of the luminance signal of the object's image represents a high contrast of the object's image, and therefore, it represents that the image is in better focus than the case of a small amount of the high-frequency component (hereinafter, an index of a focus state is represented as a focus evaluated value). This technique is so-called contrast detection focusing (described in, for example, “NHK Gijutsu Kenkyu Hokoku”, 1965, Vol. 17, No. 1, (serial No. 86), pp. 21-37). The contrast detection focusing may perform a scan operation, which moves a focus lens by a predetermined range and acquires a focus state in the predetermined range. In this case, the focus lens is moved to a position at which an object is in focus.
For a so-called continuous shooting operation, which has short time intervals between imaging operations, it is difficult to find a focus position because the time required for performing the scan operation is limited. In addition, because of various kinds of image processing, such as development and conversion for images successively captured, an enormous load is imposed on a system. Therefore, during a continuous shooting operation, the focusing is typically locked in the position of a focus lens at the first imaging operation.
Continuous shooting operations generally capture images of a moving object. This is a serious problem, considering the fact that an increased number of imaging operations in a continuous shooting operation can be technically practicable. Once the focusing has been locked, as described above, as the number of imaging operations is increased, an image of a moving object being out of focus will be captured.
To aim to address the problem, a technique of varying a scan range in a continuous shooting operation on the basis of the direction of movement of a previous focus position is described in, for example, Japanese Patent Laid-Open No. 2002-122773.
This patent document discloses varying the widths in which a scan range is divided. However, when an object moves great distances, the focus position deviates from the scan range.